The present invention relates to crosslinked aliphatic polyesters and, more particularly, in certain embodiments, to methods and compositions that use crosslinked aliphatic polyesters in well bore applications, particularly in high-temperature applications.
Aliphatic polyesters are commonly employed in well bore applications. For instance, aliphatic polyesters may be used in subterranean operations as fluid loss control particles, diverting agents, bridging agents, drilling fluid additives, cement additives, and the like. In another instance, the aliphatic polyesters may be capable of releasing a desirable degradation product, e.g., an acid, during its hydrolysis. The acid released by certain aliphatic polyesters may be used to facilitate a reduction in viscosity of a fluid or to degrade a filter cake, as well as for numerous other functions in subterranean operations.
Inclusion of an aliphatic polyester capable of releasing an acid in a gelled (and optionally crosslinked) treatment fluid may be used to facilitate a reduction in viscosity of such fluid. Generally, these aliphatic polyesters likely will hydrolyze over time due to contact with water present in the fluid, thereby releasing an acid. Upon its hydrolysis, the acid will function, inter alia, to reduce the crosslinks in the treatment fluid, reducing the pH of the treatment fluid sufficiently to reverse the crosslinks therein, and/or breaking down the backbone of the gelling agent present in the treatment fluid. Typically, the acid released by the aliphatic polyester may breakdown the gelling agents, and this phenomena may be accelerated at elevated temperatures such as those above about 150° F.
Aliphatic polyesters capable of releasing an acid may also be used in the degradation of acid-soluble materials present in a subterranean formation, such as those present in or adjacent to filter cakes. Filter cakes commonly may be formed by a fluid (e.g., a treatment fluid) on the face of a portion of a subterranean formation, inter alia, to minimize damage to the permeability thereof. The filter cake often comprises an acid-soluble component (e.g., a calcium carbonate bridging agent) and a polymeric component (e.g., starch and xanthan). Before desirable fluids, such as hydrocarbons, may be produced, the filter cake is generally removed. To facilitate the degradation of the acid-soluble component, an aliphatic polyester capable of releasing an acid may be utilized. Filter cakes also may be removed using an acid where the filter cake does not contain an acid-soluble component, for example, by degrading the underlying carbonate adjacent to the filter cake, if the filter cake is present in a carbonate formation.
In one instance, the filter cake may be contacted by a treatment fluid that comprises the aliphatic polyester. The resultant acid should interact with the acid-soluble component of the filter cake and/or the underlying carbonate adjacent to the filter cake in such a way as to facilitate their degradation. In another instance, the aliphatic polyester capable of releasing an acid may be included in the fluid (such as the treatment fluid) that forms the filter cake, such that the filter cake further contains the aliphatic polyester. Subsequent contact of the filter cake with an aqueous fluid hydrolyzes the water-hydrolysable material thereby releasing an acid that acts to degrade the acid soluble component of the filter cake. Among other components, the aqueous fluid may contain oxidizing agents or enzymes suitable to facilitate the degradation of the polymeric component of the filter cake.
The use of aliphatic polyesters may be problematic in high-temperature wells. At higher temperatures, such as those above about 300° F., the use of aliphatic polyesters in subterranean formations may not be suitable. At such temperatures, the presence of aqueous fluids could lead to early hydrolysis of the aliphatic polyesters. Similarly the use of crystalline aliphatic polyesters or stereoisomers may also be problematic in high-temperature wells.